The present invention relates, generally, to methods for analyzing lubrication oil. More particularly, the invention relates to the preparation of oil test specimens for analytical processing by an optical particle counter.
Notwithstanding the traditional perception that water and oil do not mix, in fact, water may mix with oil by three distinct mechanisms: 1) by free droplet dispersion; 2) by emulsion; and 3) by solution. At room temperature, the solubility of water in a transformer (paraffinic) type oil is about 20 ppm. The room temperature solubility of water in an aromatic (napthenic) oil is about 200 ppm.
For every 25.degree. C. temperature increase, the solubility of water in oil approximately doubles. Additionally, certain oil additives may increase the water solubility therein. Although dissolved water in a lubrication system is relatively benign, a water concentration held in dissolved dispersion at a higher temperature or chemical equilibrium may be at least partially precipitated in situ to free water by cooling or chemical change.
The presence of dispersed free water is often revealed by a hazy or cloudy appearance to the fluid. A confident but only qualitative confirmation of free water in oil is provided by a "crackle test". Free water mixed with oil "crackles" when heated above the water boiling point. The Karl Fischer reagent method, specified by ASTM D-1744, is the industry standard for quantatively measuring the presence of water in oil to an accuracy of about 10 ppm.
One of the more reliable methods for analytically determining the degree of solid particle contamination of a lubrication fluid is by means of an optical particle counter. Operatively, an optical particle counter is constructed to flow an oil specimen through a thin, transparent segment of fluid flow channel. A light source on one side of the transparent segment issues a light beam through the specimen as it flows through the transparent channel segment. On the opposite side of the transparent segment of flow channel, the light emerging from the specimen flow stream falls upon a light detector means such as a photodiode. The light detector generates signals corresponding to the size and frequency of solid particles passing through the transparent segment of flow channel while in suspension with such oil as a consequence of blocking of the light path between the light source and light detector.
When an oil specimen is also contaminated with dispersed free water, the water is distributed throughout the specimen as free droplets or as globules of emulsion. The light refractive index of these droplets or globules differs from that of the remaining fluid body. Hence, the water droplet or globule passing through the transparent segment of flow channel either blocks or disperses the light beam resulting in a report by the light detector as the passing of a solid particle. This false solid particle report vitiates the integrity of the solid particle count data.
It is, therefore, an object of the present invention to distinguish true solid particles counted by an optical particle counter from water particles.
Another object of the present invention is to mask free water in a contaminated oil specimen to an optical particle counter.
A further object of the present invention is a test sample preparation method for optical particle counters that masks free water sites in a contaminated oil specimen.